I'm looking for suggestions on an ultra low noise amplifier. Needs to be 5 watts or so, and run on 13 volts. Current draw is not an issue. Need something that will not hiss in the speaker and my Bose QCII headphones. The amplifier that is in the radio now is quite noisy.

well your headphones dont need an amp at all, they have active noise cancelling which means internal amp, feed them line level.

they are probably right though, maybe you have a different definition, but here at DIYA ultralownoise is used for high end audio preamps, dac line level stages, high performance headphone amps, their power supplies or high gain phono preamps.

for radio, though not too bad with some stations and a high quality receiver, it doesnt need this kind of performance and its unlikely you are ready for the commitment or investment needed to achieve it anyway, even if your audio source was able to support it.

your transceiver almost definitely doesnt demand it. its hard to work out if you are actually talking about an amplifier or not. you really need to provide more detail if you want help. the amplifier in the reciever? that could mean anything and you further muddied the waters by describing a situation where no amplifier is needed.

There is not much that a rig like that is used to listen to where low noise audio stages will make any difference whatsoever, the noise will be totally dominated by some combination of sky noise and things like reciprical mixing products, not much anything down at audio can do about those....

Best way to get lower noise is to use the narrowest bandwidth you can, best aerial system you can build and after that, move to a location with lower QRM, get a tower and a rather large beam.

Seriosuly, while the audio stages in a comms RX are usually pretty much an afterthought, they are seldom the limiting factor in terms of the noise floor.